Though present in natural settings at minute quantities, ethylene oxide (“EO”) was first synthesized in a laboratory setting in 1859 by French chemist Charles-Adolphe Wurtz using the so-called “chlorohydrin” process. However, the usefulness of ethylene oxide as an industrial chemical was not fully understood in Wurtz's time; and so industrial production of ethylene oxide using the chlorohydrin process did not begin until the eve of the First World War due at least in part to the rapid increase in demand for ethylene glycol (of which ethylene oxide is an intermediate) as an antifreeze for use in the rapidly growing automobile market. Even then, the chlorohydrin process produced ethylene oxide in relatively small quantities and was highly uneconomical.
The chlorohydrin process was eventually supplanted by another process, the direct catalytic oxidation of ethylene with oxygen, the result of a second breakthrough in ethylene oxide synthesis, discovered in 1931 by another French chemist Theodore Lefort. Lefort used a solid silver catalyst with a gas phase feed that included ethylene and utilized air as a source of oxygen.
In the eighty plus years since the development of the direct oxidation method, the production of ethylene oxide has increased so significantly that today it is one of the largest volume products of the chemicals industry, accounting, by some estimates, for as much as half of the total value of organic chemicals produced by catalytic heterogeneous oxidation. Worldwide production in the year 2000 was about 15 billion tons. (About two thirds of the ethylene oxide produced is further processed into ethylene glycol, while about ten percent of manufactured ethylene oxide is used directly in applications such as vapor sterilization.)
The growth in the production of ethylene oxide has been accompanied by continued intensive research on ethylene oxide catalysis and processing, which remains a subject of fascination for researchers in both industry and academia. Of particular interest in recent years has been the proper operating and processing parameters for the production of ethylene oxide using so-called “high selectivity catalysts”, that is Ag-based epoxidation catalysts that contain small amounts of “promoting” elements such as rhenium and cesium.
Catalytic oxidation of ethylene to ethylene oxide is usually practiced as a gas phase process in which the feed is contacted in the gas phase with the catalyst present as a solid material. The catalyst is typically positioned in a tubular, packed-bed reactor, and the reactor is typically equipped with heat exchange facilities to heat or cool the catalyst. Rather than directly measuring the catalyst temperature, the temperature of the process is typically determined by measuring the “coolant temperature” that is, the temperature of the coolant outside the tubes.
Halogen-containing compounds, especially chlorohydrocarbons, have long been used in the feed mixture for the gas phase production of ethylene oxide (see e.g., Law et al., U.S. Pat. No. 2,279,469, issued Apr. 14, 1942; U.K. Patent No. 1,055,147 issued Jan. 18, 1967, and Lauritzen, EPO Patent No. 0 352 850 B1, issued Jan. 19, 1994). The added halogen-containing compound has been variously known as an “inhibitor”, “modifier”, “moderator”, “anti-catalyst”, and “promoter”, and is herein called a “moderator”.
The moderator plays a key role in maintaining the catalyst's activity and selectivity for producing ethylene oxide. This is especially true for rhenium-containing, highly selective catalysts where optimum performance can only be obtained within a narrow moderator concentration range within the feed mixture. Furthermore, this optimum moderator concentration range is not fixed, but it changes with temperature. Catalyst performance deteriorates with time, so temperature is generally increased with time to maintain a constant rate of ethylene oxide production. The moderator concentration must therefore be incrementally increased with temperature to keep the catalyst operating at peak efficiency.
The highly selective catalyst's efficiency for producing ethylene oxide in catalytic oxidation of ethylene is very important. This efficiency is a combination of catalyst selectivity and catalyst activity. Selectivity is defined as the amount of ethylene oxide produced for a given amount of ethylene (or oxygen) reacted on the catalyst; whereas, the activity is customarily expressed in terms of the reactor coolant temperature required for production of ethylene oxide at a given rate. The rate of ethylene oxide production is commonly expressed in terms of the amount of ethylene oxide produced per unit volume (or mass) of the catalyst per unit time.
Because the selectivity and activity of the highly selective catalyst are both very sensitive to the concentration of moderator in the reactor feed, the moderator concentration must be carefully tuned to maximize the efficiency of the catalyst. Historically, operators of the highly selective catalyst have attempted to optimize the moderator concentration by trial and error. The skilled operator would make an incremental change to the moderator concentration, up or down, and then wait to see the change in catalyst efficiency. If catalyst efficiency improved, then the operator would continue making incremental changes in the same direction until maximum ethylene oxide selectivity could be obtained at the lowest reactor coolant temperature. If catalyst efficiency had not improved with the change in moderator concentration, then the operator had to reverse the steps and attempt to optimize catalyst efficiency by moving moderator concentration in the opposite direction. This optimization process is painstakingly slow and tedious and generally must be executed by someone skilled in the art of operating the highly selective catalyst. The optimization process can be especially difficult or impossible if the temperature is fluctuating.
Increasing the moderator concentration above the optimum generally causes selectivity to decrease, but because the catalyst function may degrade more quickly at higher temperatures, it is sometimes desirable to increase the moderator concentration still further, anyway, and to suffer some selectivity loss in exchange for operating the catalyst at a lower temperature.
Temperature can increase or decrease when a change is made to the operating conditions of the catalytic oxidation of ethylene to ethylene oxide process. Temperature is generally increased over the service life of the catalyst to compensate for the loss in the catalyst's activity. Irrespective of the cause, the moderator concentration must be re-optimized every time the temperature changes. Again, this means making small adjustments to the moderator concentration until it appears that maximum catalyst operating efficiency has been reestablished. Even for persons skilled in the art, these repetitive, incremental re-optimizations are difficult and make it inherently difficult to keep the catalyst operating at peak efficiency and likewise, to maintain high efficiency in the overall catalytic process.
The present invention solves this problem by precisely correlating the change in moderator concentration with the change in temperature to keep the highly-selective catalyst operating at peak efficiency in the oxidation of ethylene to ethylene oxide. With the inventive correlation, there is no longer need to search for the new optimum by a skilled operator, making iterative, incremental changes to the moderator concentration, hoping to find the new optimum. When the temperature changes, the moderator concentration in the feed is simply adjusted, manually or automatically, to the new level given by the inventive correlation. This action can be completed by any operator, because it requires no special skill.
In fact, the invention is sufficiently analytic that it can be automated or controlled by a digital control system. Techniques for such automation of the moderator levels have been proposed in the prior art. For example, U.S. Pat. Nos. 7,657,331 and 7,657,332 recite specific formulas and ratios to predict what the optimal modifier (herein, moderator) levels should be, making use of a “Q value” for calculating the correct chloride concentration. This Q value is the ratio of the total “effective” moderator to the total “effective” hydrocarbon. The “effective” hydrocarbon value is determined by multiplying the molar concentration for each species of hydrocarbon by a correction factor that (according to theory) accounts for the differences in the ability of the different hydrocarbons to remove/strip reaction modifier from the surface of the catalyst; while the “effective” moderator value is determined by multiplying the molar concentration for each species of moderator by a correction factor that (again, according to theory) accounts for the number of “active species” present in a specific moderator. These correction factors are determined for each individual hydrocarbon and moderator by what is, apparently, a complicated process of experimental trial and error; however, the process for determining these correction factors is not set out with specificity in the aforementioned patents nor any actual examples of the procedure presented.
Within this same prior art, it is also taught that “when the reaction temperature is increased or decreased, the position of the selectivity curve for the modifier [moderator] shifts towards a higher value of Q or a lower value of Q, respectively, proportionally with the change in the reaction temperature.” Similarly, U.S. Pat. Nos. 7,102,022 and 7,485,597 also teach that “deviations from the optimum selectivity which would result from a change in temperature may be reduced or even prevented by adjusting the value of Q proportionally with the change in catalyst temperature.” These four examples teach that Q must be adjusted in a linear fashion with temperature according to the relationship:Q2=Q1+B(T2−T1)where T is temperature, Q is the ratio of the total effective moderator to the total effective hydrocarbon and B is the linear proportionality constant.
The present invention shows that optimum moderator concentration does not vary in a linear fashion with temperature. For a highly selective catalyst in the catalytic oxidation of ethylene to ethylene oxide, the relationship between optimum moderator concentration and temperature shows distinct curvature and is predominantly exponential. The present invention accounts for this curvature and therefore overcomes the inadequacy of the prior art. The present invention can now be used in straightforward manner to maintain optimum selectivity when the temperature of the catalytic process changes.
For maintaining optimum moderator concentration, the cases cited from the prior art require that correction factors for each species of moderator and hydrocarbon species in the feed gas be determined and also requires these correction factors be used in detailed calculations for determining Q. Aside from the values of correction factors for the hydrocarbons and moderators that are specifically set forth in the specification, there is no general procedure detailed in the description to determine the correction factors for the hydrocarbons and moderators. Moreover, it is not clear how broadly the measurement of correction factors may be applied across the variance in operating parameters and circumstances that are experienced under actual service conditions for the highly selective catalyst. Specifically, it is not clear if a correction factor measured with respect to a first composition of reaction modifiers and hydrocarbons and a first catalyst and catalyst surface, can be subsequently used later with a different composition of reaction modifiers and hydrocarbons and a different catalyst and catalyst surface. On yet a more general level, there is no evidence to support the theory that is the basis for these patents regarding the correlation of correction factors to the stripping behavior of hydrocarbons, and the ability of moderator actives to “split” off from compounded heteroatoms. The present invention overcomes these apparent limitations, requiring neither determination of correction factors nor calculation of Q.
EP Patent No. 0 352 850 B1 teaches that “after the catalyst has ‘lined-out’ and normal operating conditions are reached,” that the “chlorohydrocarbon moderator [be] slowly increased over the run time at an average rate of increase of at least 0.5% per month during the operation of the catalyst, more preferably at an average rate of increase of at least 1% per month and even more preferably at an average rate of increase of at least 3% per month and yet even more preferably at an average rate of increase of at least 5% per month.” While EP 0 352 850 prescribes the need to increase the concentration of the moderator in the feed with operation time, it fails to provide an approach for the critical element of maintaining optimum moderator concentration with temperature change.
According to the prior art, maintaining the most preferred moderator levels is a laborious and time consuming process. It would require considerable expertise among operational staff, but such staff expertise is not always available, particularly in commercial, continuously operating ethylene oxide plants. Rather than requiring such expertise to always be available, it would be highly desirable to develop a process for controlling moderator levels that is sufficiently analytic that it can automated or even programmed into a digital control system.
Given the disadvantages of the currently practiced methods, as well as the importance of maintaining proper moderator level to the performance of the highly selective catalysts, there is a continuing need for a process for controlling moderator levels that is sufficiently analytic that it can be automated or programmed into a digital control system and yet can be practically implemented at plant scale without the need for extensive and speculative empirical manipulation. This is accomplished in the present invention.